you might have a problem with your crowns or stem rotting. rockwool holds a lot of moisture and you'd be providing it with a constant source to wick it up.
As to your actual question; how well it grips will probably depend on what you're growing. young plants and light plants I don't see being a problem, but something with some weight might be a horse of a different color.

On your first sentence, do you mean that it is generally not a good idea to use rockwool in raft culture? If so, what would be a good alternative?

As for my original question, I actually based it on this video on youtube.
link:
https://www.youtube.com/watch?v=m45vMicFtuE

They use NFT there, so I'm wondering if anyone has experience doing this in raft :)

Don't get me wrong. I don't use Rockwool, but a lot of people have problems with stem rot because rockwool holds so much water.
In a raft system, the rockwool will stay in contact with the water thereby always staying saturated.
with NFT, it depends on whether the rockwool contacts the flow or not. if it does, I would suspect he same problems, if not, then it probably wouldn't. I use net pot in nft and they don't touch the flow and the top hydroton never looks wet.

i see, do you think this also holds true with coco coir?

I imagine you would have the same problem. I've only used coir in an EnF though so don't know specifically about a raft. In the raft I made, I used hydroton.